[RFC PATCH 0/3] Use pageblock_order for cma and alloc_contig_range alignment.

Vlastimil Babka
Tue, 23 Nov 2021 17:32:37 -0000

On 11/23/21 17:35, Zi Yan wrote:
> On 19 Nov 2021, at 10:15, Zi Yan wrote:
>>>> From what my understanding, cma required alignment of
>>>> max(MAX_ORDER - 1, pageblock_order), because when MIGRATE_CMA
was introduced,
>>>> __free_one_page() does not prevent merging two different
pageblocks, when
>>>> MAX_ORDER - 1 > pageblock_order. But current
__free_one_page() implementation
>>>> does prevent that.
>>>
>>> But it does prevent that only for isolated pageblock, not CMA, and
yout
>>> patchset doesn't seem to expand that to CMA? Or am I missing
something.
>>
>> Yeah, you are right. Originally, I thought preventing merging isolated
pageblock
>> with other types of pageblocks is sufficient, since MIGRATE_CMA is
always
>> converted from MIGRATE_ISOLATE. But that is not true. I will rework the
code.
>> Thanks for pointing this out.
>>
>
> I find that two pageblocks with different migratetypes, like
MIGRATE_RECLAIMABLE
> and MIGRATE_MOVABLE can be merged into a single free page after I checked
> __free_one_page() in detail and printed pageblock information during buddy
page
> merging.

Yes, that can happen.

I am not sure what consequence it will cause. Do you have any idea?

For MIGRATE_RECLAIMABLE or MIGRATE_MOVABLE or even MIGRATE_UNMOVABLE it's
absolutely fine. As long as these pageblocks are fully free (and they are if
it's a single free page spanning 2 pageblocks), they can be of any of these
type, as they can be reused as needed without causing fragmentation.

But in case of MIGRATE_CMA and MIGRATE_ISOLATE, uncontrolled merging would
break the specifics of those types. That's why the code is careful for
MIGRATE_ISOLATE, and MIGRATE_CMA was until now done in MAX_ORDER granularity.